Fix shell shadows being offset

[thnewlands]

Shell shadows get clipped near the camera or are too far offset in depth with custom depth being set. I've changed the way they're calculated to be more consistent with the way that the fragment function works in URP. https://github.com/needle-mirror/com.unity.render-pipelines.universal/blob/a158b7a53654edcf071beab1016cdcbc8fe8aff1/Shaders/ShadowCasterPass.hlsl#L59